This is the Hardest Question
in TF1: when the Autobots Arrive and they all transform for the first time
in TF2: When Optimus Fight the Fallen and Megatron
in: TF3 The Whole Ending Battle and the highway scene

1: Optimus vs Megartron. (C'mon, it may have been short, but it's my one of my favorite rivalries of my childhood relived on the big-screen!)
2: Forest Battle. (See the top explanation. ^)
3. The battle in Chernobyl. (Not sure why, I guess seeing my favorite villain on the big-screen made it so great.)